5.2 DNA Replication
SBI4U1
Humans rely on the continual regeneration of
cells ( ie. Especially during injury)
– Examples:
• Humans begin as a single fertilized egg
• Red blood cells, skin cells must regenerate continuously
The production of new cells is achieved through
Mitosis & Cytokinesis
Recall: Cell Cycle
CELL CYCLE
1) Growth stage:
Interphase
2) Cell division:
production of 2
daughter cells
*note that DNA is
copied during S
phase of
interphase in the
cell cycle.
• In order for cells to divide they must:
– Grow
– Carry out metabolic activity
– Replicate DNA
• During division, each daughter cell contains
exact same genetic material as parent cell
DNA Replication: process of producing two
identical DNA molecules from an original,
parent DNA
There are 3 proposed models for DNA Replication:
1. Semiconservative
2.Conservative
3.Dispersive
Models of DNA
Replication
1) Conservative model :
results in one new
molecule and
conserves the old
2) Semi-conservative
model: results in two
hybrid molecules of
old and new strands.
3) Dispersive model :
results in hybrid
molecules with each
strand being a
mixture of old and
new strands.
Now sit back and relax….
http://www.youtube.com/watch?v=TCuxp7tgb0g
Meselson and Stahl
• Used different isotopes of N to label DNA in a cell
– 14N (common) and 15N (rare, aka “heavy”)
– Nitrogen was chosen b/c its an integral part of
DNA
– They were able to separate densities of isotopes
using centrifugation( more dense =sink to bottom,
less dense= stay on top)
The Experiment:
• They grew E. coli bacteria in a medium with 15N for 17
generations
• b/c N is in DNA, both strands of bacteria are
labeled with 15N
• Some bacteria were transferred to a medium
with 14N and reproduced
Observations:
• DNA samples from 15N were uniform
• After 1st round of replication DNA was single
band intermediate 15N and 14N
• After 2nd round of replication DNA was two
band, half 15N and 14N and half 14N
• After more replication, two bands, similar to
prior – supports SEMI CONSERVATIVE!!
Homework:
• Read and make notes 5.2
• Complete Learning Check
pg. 222 #13-18
DNA REPLICATION
Initiation-Elongation-Termination
Recall:Semi-Conservative Replication
• Mechanism of DNA replication that produces
two copies
– both are made up of one new strand and
one from original DNA ( parent strand)
• There are 3 basics phases in replication:
1. Initiation
2. Elongation
3. Termination
Even though DNA replication is semi-conservative, due to
complimentary base pairing, the two copies of DNA are
identical.
1. Initiation
• A portion of DNA double helix is unwound to
expose bases for new base pairing
• It begins at the origin of replication (specific
nucleotide sequence).
• This is where the unwinding begins to form a
“bubble” and as a result two replication forks are
formed
As replication proceeds, each replication fork
moves along the DNA in opposite directions.
Replication
Fork
Initiation cont’d… Important Enzymes
• Helicase ( enzyme responsible for the
unwinding of the double helix) It cleaves Hbonds b/t complimentary base pairs
• Other enzymes(SSBP) ensure new single
strands do not “re-wind” into double helix
• Topoisomerase II helps to relieve strain on
double helix section above replication fork
2. Elongation
Synthesis of 2 new DNA strands, each composed
of 1 parent strand
• DNA Polymerase III
– Enzyme that adds new nucleotides to create
complementary strand of DNA
– Begins on the free 3’ hydroxyl end of parent
strand
– Moves in 5’ to 3’ direction toward replication fork
(This is called the “leading strand”
Lagging Strand  DNA polymerase III moves in
opposite direction, away from replication
RNA Primer is short strand of RNA that is used to
start or “prime” DNA replication
– Primase is an enzyme that synthesizes RNA primer
– Several primers are used to build lagging strand
– Once each primer is added, new DNA fragments are
generated ( these are called OKAZAKI fragments)
• DNA Polymerase I removes RNA primer, fills
gaps b/t Okazaki fragments
• DNA ligase joins the Okazaki fragments
together
Okazaki Fragments
3.Termination
• Newly formed strand automatically rewind
into double helix after replication
• The new DNA strands separate from each
other
Animation: http://highered.mcgrawhill.com/sites/dl/free/0072437316/120076/bio23.swf
Repairing Replication Errors
• Entire human DNA can be copied in a few hours
-Errors are likely to occur, as so much is happening at
once
• Examples of Errors:
1) Mispairing b/t 2 nucleotides ( ie. T paired wit G)
2) Strand slippage causing additions/omissions of
nucleotides
1) Mispairing
Mispairing is when
complimentary base
pairs to not match up.
For example, G joins
up with T.
This is due to
flexibility in DNA
structure and can
accommodate
misshaped pairings.
2) Strand Slippage
How do we fix errors in DNA
Replication??
1) DNA Polymerase II enzymes “proofread” the
newly synthesized DNA to catch any errors
• Catches ~99% of mismatch
2)Mismatch repair
– A group of enzymes recognize mispaired
nucleotides on the new strand
– Replace it with correct nucleotide
…Beyond this, mutations can result
Comparing DNA Replication (pg. 227)
Prokaryotes
Eukaryotes
***Use your textbook to complete the Venn diagram
comparing DNA replication in Prokaryotes and Eukaryotes**
Learning Expectations...
•
•
•
•
Purpose of DNA replication
Different models for DNA replication
Meselson and Stahl’s Experiment & main findings
Semi-Conservative Replication
– Initiation
– Elongation (leading vs. lagging strand/Okazaki fragments)
– Termination
•
•
•
•
•
Diagram of DNA replication
All enzymes involved in DNA replication
Errors in DNA replication
Mechanisms of repair for replication errors
Compare/contrast DNA replication in prokaryotes and
eukaryotes
Homework
• Pg. 229 # 3, 5, 6, 7, 8, 10, 11
• Quiz to follow on Chapter 5
• Review h/w for Chapter 5:
• pg. 235-236
pg. 238-239
# 1-14 ,16, 18-28
# 48, 52,72